The characteristic strength of concrete is defined as that compressive strength below which NOT more than:

📊 Definition of Characteristic Strength

The characteristic compressive strength (f_ck) of concrete is defined in IS 456:2000 as the value of compressive strength below which not more than 5% of test results are expected to fall. In other words, 95% of all results should lie at or above this value.

This statistical definition means that when you specify M25 concrete, you accept that up to 1 in 20 cubes may fall below 25 N/mm² — hence the need for a target mean strength that is higher than f_ck.

Key takeaway

Characteristic strength = 5th percentile value. 95% of results lie above it. This is why the target mean must be set higher than f_ck.